Stability and crystal chemistry of iron-bearing dense hydrous magnesium silicates

Dense hydrous magnesium silicates (DHMS) are supposed to be key phases in planetary water cycles because of their ability to carry water to deep mantle regions in subduction slab environments. In order to understand water cycles in iron-enriched planetary systems such as Mars knowledge of the water content and stability of iron-bearing DHMS is required. Iron-bearing DHMS were synthesized based on two starting compositions, MgFeSiO₄ + 9.5 wt% H₂O system and a simple hydrous Martian mantle composition containing Fe, Mg, Al and Si + 12.35 wt% H₂O (hydrous FMAS system). Compared to literature data on phase D, iron-bearing phase D shows analogous variations in water contents as Mg-phase D but appears to be stable at higher temperatures than Mg-phase D for both starting compositions used in this study. Iron-bearing superhydrous phase B contains up to 7 wt% H₂O and shows an extended thermal stability in the hydrous FMAS system. The high-temperature stability of iron-bearing DHMS with a Mars-like bulk composition indicates that these hydrous phases could host significant amounts of water at core-mantle boundary conditions (1500 °C and 23 GPa) in a hydrous Martian mantle.     

Thermodynamic properties of MgSiO3 ilmenite from vibrational spectra

Unpolarized infrared (IR) reflectance spectra for MgSiO₃ ilmenite taken from a single-crystal and from a densly packed polycrystalline sample possessed all eight peaks mandated by symmetry between 337 and 850 cm^?1. Polarizations were inferred from intensity differences between the two samples. IR peak positions differ by up to 250 cm^?1 from recent calculations, but on average are within 11%. Heat capacity C _p calculated from these data by using a Kieffer-type model are within the experimental uncertainty of calorimetric measurements from 170 to 700 K. Outside this range, calculated C _p is probably accurate within a few percent, based on recent results for garnets. Calculated entropy is only slightly less accurate, giving S ⁰ (298.15 K) as 54.1 ±0.5 J/ mol-K, which is 10% lower than recent estimates based on phase equilibria. The slope of the phase boundary between ilmenite and perovskite is used to predict S ⁰ (298.15 K) of perovskite as 58.7 ±1.4 J/mol-K, which is 10% lower than previous values.     

Far infrared spectra of hydrous layer silicates

Observation of major bands seen in infrared spectra of 26 phyllosilicates (23 of which were produced in the laboratory) are reported for wave numbers from 50 to 280 cm^?1. Substitutions in the various structural sites (interlayer, tetrahedral and octahedral) permit one to identify the ions which contribute to the vibrations which give rise to bands in the infrared spectra. No attempt is made to assign vibrational modes or specific vibrational types. Using the following ion substitutions, OH-OD; Na-K-Sr-Mg-Ca; Si-Ge; Al-Ga; Mg-Co-Ni-Fe, it is apparent that in the 7 Å chlorite (amesite and chrysotile), kaolinite, pyrophyllite, aluminous dioctahedral mica, aluminous smectites and trioctahedral micas it is not possible to attribute any low frequency bands as being dominated by interlayer ion stretch vibrations (alkali ions). The cations which participate in the vibrators responsible for the dominant modes observed then seem to be Si and Al. This does not exclude the existence of interlayer ion stretch modes in these spectral regions, however they could not be identified. In the materials studied only a few bands can be attributed to hydroxyl-related vibrations and little influence is seen for octahedrally coordinated ions in dioctahedral minerals. It is important to note that the lowest frequency bands (80–140 cm^?1) are apparently dominated by vibrations in the network and especially to the Si-O part of the structure. Low frequency bands are however most apparent in charged layer structures, i.e. micas and smectites.     

29Si and 1H NMR spectroscopy of high-pressure hydrous magnesium silicates

We present NMR spectroscopic data, obtained by ¹H MAS, ¹H static spin-echo, and ²⁹Si{¹H} CP-MAS techniques, for a series of hydrous magnesium silicate samples synthesized at high pressure. This series includes chondrodite, β-Mg₂SiO₄, and phases A, B, superhydrous B, and E. Phases B and superhydrous B give very narrow ²⁹Si NMR peaks and display the most de-shielded Si^VI chemical shifts yet reported: ?170.4?ppm for B and ?166.6 for superhydrous B. The ¹H NMR spectra of B and superhydrous B confirm the presence of paired hydroxyls, as determined from refinement of the H positions from X-ray diffraction data. The ¹H MAS NMR spectra of phase B contain peaks for the two distinct hydrogen positions, with chemical shifts of +4.7 and +3.3?ppm. The static ¹H spectrum contains a powder pattern characteristic of a strongly coupled hydrogen pair, from which a dipolar coupling constant of 18.6(4)?kHz and inter-hydrogen distance of d(H–H)=1.86(2)?Å were obtained. Superhydrous B appears to give two poorly resolved ¹H MAS peaks, consistent with the presence of two distinct hydrogen pairs in the P2₁ mn crystal structure. Analysis of its spin-echo spectrum gives d(H–H)=1.83(3)?Å, slightly shorter than for phase B. β-Mg₂SiO₄, coexisting with phases B and superhydrous B, appears to give ²⁹Si{¹H} CP-MAS signal, indicating that it contains significant H concentration. The ²⁹Si chemical shifts for phases B, superhydrous B, and chondrodite, together with those reported previously for other Mg-silicates, show a good correlation with structural parameters.     

Infrared spectra of high-pressure hydrous silicates in the system MgO-SiO2-H2O

Infrared absorption spectra are presented for four hydrous magnesian silicates which are stable at high pressures and high temperatures characteristic of the earth's mantle: phase A (Mg₇Si₂O₁₄H₆), phase B (approx. Mg₂₃Si₈O₄₂H₆), chondrodite (Mg₅Si₂O₁₀H₂), clinohumite (Mg₉Si₄O₁₈H₂). The results show that phase B as well as phase A, chondrodite and clinohumite contain hydroxyl groups in their structures. It is also suggested that phase B, in addition to phase A, could be of potential importance in the upper mantle and transition zone as a dense host mineral for hydroxyl ions. Chemical composition of phase A was reexamined by microprobe analysis: it agrees well with Mg₇Si₂O₁₄H₆, estimated by Yamamoto and Akimoto (1974, 1977), but disagrees with a newly proposed formula Mg₂SiO₅H₂ by Benimoff and Sclar (1984).     

Sulfide and platinum mineralization in the Merensky Reef: evidence from hydrous silicates and fluid inclusions

The base metal sulfides of the Merensky Reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates. Biotite and phlogopite crystallized in the vicinity of sulfides from a volatile-enriched highly fractionated intercumulus melt. Amphibole, chlorite, and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase. Biotite is often accompanied by zircon, rutile, and quartz. Accessory quartz hosts a complex suite of H₂O-NaCl-(CaCl₂)-CO₂-CH₄ fluid inclusions which have thus far not been described from the Merensky Reef. The earliest fluid inclusion compositions are NaCl-(H₂O) with less than 10 vol.% water; CO₂ coexisting with a halite daughter crystal and brine; and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations. The maximum trapping temperature is around 730° C at 4 to 5 kb pressure. Later inclusion generations are H₂O-NaCl, CO₂-H₂O, and pure CO₂ and CH₄. The presence of Cl-rich fluids during the intercumulus stage of the crystallizing Merensky Reef is directly related to the mode of sulfide precipitation. Prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as H₂S. Sulfur saturation causes reaction of H₂S with oxides of the silicate melt to form a sulfide melt plus water. During reaction the magma is enriched in water until a separate fluid unmixes. It carries all compounds with high fluid/melt partition coefficients, as well as metals capable of forming OH- and Cl-complexes. Precious metals are assumed to have fractionated into the Cl-rich fluid as Cl-complexes rather than being dissolved in the sulfide melt. During the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets. The model proposed explains the distribution pattern of platinum-group minerals in the Merensky Reef better than any orthomagmatic mineralization concept offered so far.     

Thermodynamic analysis of binary liquid silicates and prediction of ternary solution properties by modified quasichemical equations

Modified quasichemical equations, developed for the analysis of the thermodynamic properties of structurally ordered liquid solutions, are shown to be well-suited for use with molten silicates. For binary systems, these equations have been coupled with a least-squares optimization computer program to analyse simultaneously all thermodynamic data including phase diagrams, Gibbs energies and enthalpies of formation of compounds, activities, enthalpies of mixing, entropies of fusion, miscibility gaps, etc. In this manner, data for several binary systems have been analysed and represented with a small number of parameters. In the present article, results for the SiO₂-MgO, SiO₂-Na₂O and MgO-CaO systems are presented. The resulting equations represent all the binary data, including the phase diagrams, within or virtually within experimental error limits.From the modified quasichemical equations for ternary systems, ternary thermodynamic properties can be approximated solely from data from the subsidiary binary systems. Results for the SiO₂-CaO-Na₂O, SiO₂-CaO-MgO, and SiO₂-MgO-FeO systems are in excellent agreement with measured ternary data. Predictions for the quaternary system SiO₂-MgO-CaO-Na₂O are also presented.     

The composition of liquids coexisting with dense hydrous magnesium silicates at 11-13.5 GPa and the endpoints of the solidi in the MgO-SiO2-H2O system

High-pressure liquids in the MgO-SiO₂-H₂O (MSH) system have been investigated at 11 and 13.5 GPa and between 1000 and 1350 °C. A bulk composition more magnesian than the tie-line forsterite-H₂O was employed for the study. Rocking multi-anvil experiments were combined with a diamond trap set-up. After termination of the experiments, the liquid trapped in the diamond layer was analysed by laser ablation ICP-MS using the ‘freezing’ technique. At 11 GPa, liquids coexist with one or two of phase A, clinohumite, chondrodite, and forsterite. A marked discontinuity in the evolution of liquid compositions near 1100 °C is observed at 11 GPa. A step of ∼13 wt% H₂O and 13 wt% MgO is interpreted to result from overstepping the fluid-saturated solidus reaction mass balanced to 1.00(18) phase A + 1.07(4) fluid = 0.63(15) chondrodite + 1.44(2) melt. At 13.5 GPa liquids coexist with one or two of hydrous wadsleyite, clinohumite, superhydrous B, phase B, and forsterite. The discontinuity in liquid composition is no longer present, indicating that the second critical endpoint of the solidus has been overstepped. Thus, hydrous melts in the Mg-rich part of the MSH system (molar bulk Mg/Si > 2) are chemically distinct from aqueous fluids at pressure up to 11 GPa. Convergence of fluid and melt compositions along the solidus resulting in a supercritical liquid occurs between 11 and 13.5 GPa, at which pressure the entire MSH system becomes supercritical.     

Thermodynamic properties and equation of state of fcc aluminum and bcc iron, derived from a lattice vibrational method

We use a lattice vibrational technique to derive thermophysical and thermochemical properties of the pure elements aluminum and iron in pressure–temperature space. This semi-empirical technique is based on either the Mie–Grüneisen–Debye (MGD) approach or an extension of Kieffer’s model to incorporate details of the phonon spectrum. It includes treatment of intrinsic anharmonicity, electronic effects based on the free electron gas model, and magnetic effects based on the Calphad approach. We show that Keane’s equation of state for the static lattice is better suitable to represent thermodynamic data for aluminum from 1 bar to pressures in the multi-megabar region relative to Vinet’s universal and the Birch–Murnaghan equation of state. It appears that the MGD and Mie–Grüneisen–Kieffer approach produce similar results, but that the last one better represents heat capacity below room temperature. For iron we show that the high temperature behavior of thermal expansivity can be explained within the Calphad approach by a pressure-dependent Curie temperature with a slope between –1 and 0 K/GPa.     

High temperature infrared spectra of hydrous microcrystalline quartz

A series of in-situ high temperature infrared (IR) measurements of water in an agate sample and in a milky quartz has been conducted in order to understand the nature of water in silica at high temperatures (50–700?°C) and the dehydration behavior. IR absorption bands of water molecules trapped in the milky quartz showed a systematic decrease in intensities and a shift from 3425?cm^?1 at 50?°C toward 3590?cm^?1 at 700?°C without any loss of water. This indicates a change in IR absorption coefficients corresponding to different polymeric states of water at different temperatures. The broad 3430?cm^?1 band in the agate sample also showed a systematic decrease in IR intensity and a band shift toward higher frequency with increasing temperature (～700?°C). This indicates that the agate sample also contains fluid inclusion-like water. For this agate sample, a dehydration of loosely hydrogen-bonded molecular water occurred at lower temperatures (<200?°C). At higher temperatures (>400?°C), sharp bands around 3660 and 3725?cm^?1 (3740?cm^?1 at 50?°C) due to surface silanols, appeared. This indicates dehydration of H₂O molecules that are hydrogen bonded to surface silanols. SiOH species in the agate are divided into three groups, namely SiOH group located at structural defects, surface silanols hydrogen bonded to each other and free surface silanols. Former two dehydrate below 700?°C and the dehydration rate of the SiOH at structural defects is faster than the other. IR spectra show that SiOH species decrease continuously even after the dehydration of most of H₂O molecules. All these results provide realistic bases for the change in physicochemical states of different OH species in silica at high temperatures.     

Cation order in olivines: Evidence from vibrational spectra

A simple theory is developed which relates cation ordering in the olivine structure to compositional trends in the vibrational spectra of olivines. Quantitative results for (Mg, Mn) and (Fe, Mn) olivines indicate that Mn²⁺ favors the M2 site relative to both Mg²⁺ and Fe²⁺.     

Equations of state of magnesium silicates anhydrous B and superhydrous B

High-pressure single crystal X-ray diffraction experiments of phase anhydrous B and superhydrous B have been carried out to 7.3 and 7.7?GPa, respectively, at room temperature. Fitting a third-order Birch-Murnaghan equation of state to the P-V data yields values of V ₀?=?838.86?±?0.04?ų, K_T,0?=?151.5?±?0.9?GPa and K′?=?5.5?±?0.3 for Anhy-B and V ₀?=?624.71?± 0.03?ų, K_T,0?=?142.6?±?0.8?GPa and K′?=?5.8?±?0.2 for Shy-B. A similar analysis of the axial compressibilities in Anhy-B reveals that the c-axis is most compressible (K_c?=?137?±?3?GPa), the b-axis is least compressible (K_b?=?175?±?4?GPa), and the a-axis is intermediate (K_a?=?148?±?1?GPa). In Shy-B, the a-axis is most compressible (K_a?=?135?±?1?GPa), followed by the b- and c-axes which have similar compressibilities (K_b?=?146?±?3?GPa; K_c?=?148?±?3?GPa). The fact that the b-axis of Shy-B is approximately 16% more compressible than Anhy-B is primarily due to differences in the O-T layer in which the H atoms are located and the linkages with the adjacent O layers. The rigid edge-sharing chains of MgO₆ and SiO₆ octahedra in the O layer control compressibility along the a- and c-axes in both structures. The net result is a reduction in the overall anisotropic compression from ～22% in Anhy-B to ～9% in Shy-B.     

The electrical conductivity of some hydrous and anhydrous molten silicates as a function of temperature and pressure

Electrical conductivity of the following molten silicate systems (in mol%): 60SiO₂-40Na₂O; 65SiO₂-35Na₂O; 75SiO₂-25Na₂O; 78SiO₂-22Na₂O; 72SiO₂-24Na₂O-4CaO; 66SiO₂-19Na₂O-15H₂O; and an anhydrous and hydrous (4 wt% H₂O) Mt. Erebus lava, have been measured as a function of temperature (to 1000°C) and pressure (to 1.3 kbar). The anhydrous soda-rich melts have a positive pressure coefficient of conductivity to ~200 bars and beyond this pressure the pressure coefficient is small and negative. Addition of water lowers the conductivity and gives rise to a negative pressure coefficient at the highest temperatures. The conductivity of hydrous Mt. Erebus lava passed through a maximum with increasing temperature at constant pressure. These phenomena are interpreted in terms of explanations of similar phenomena found in molten salt and aqueous electrolyte solutions.     

Physical properties of calcium aluminates from vibrational spectroscopy

Heat capacity (C_V) and entropy (S) as a function of temperature were calculated for phases in the CaO-Al₂O₃ system from vibrational spectra using a quasi-harmonic model. Calculated values of C_V at 298 K for the calcium aluminates may have uncertainties as small as ±1%, based on comparison with published calorimetric data for CaO, Al₂O₃, and CaAl₂O₄. For hibonite (CaAl₁₂O₁₉), we predict C_P as 519.3 J/mol-K and S as 391.7 J/mol-K, at 298 K. For grossite (CaAl₄O₇), calculated values of C_P as 195.9 J/mol-K and of S as 172.0 J/mol-K are slightly smaller than the available calorimetric data at 298 K, consistent with calorimetric data having been obtained from samples containing ∼10 wt% hibonite impurities. Thermal conductivity at 298 K (k₀) is predicted from peak widths of the vibrational modes using the damped harmonic oscillator model of a phonon gas. Calculations of k₀ for CaO and Al₂O₃ differ from the measurements by 17% and 5%, respectively. The discrepancy for lime is larger due to uncertainties in its peak widths. This comparison suggests that our results for the calcium aluminates should be more accurate than conventional measurements of k₀ which are commonly uncertain by ∼25%.     

Fully automated microcomputer calculation of vibrational spectra

A fully automated method for computing frequencies and atomic displacements of normal modes, giving synthetic infrared and Raman spectra, has been developed for use on small computers. No expertise in group theory or the mathematics of normal-mode calculation are required to use the computer program. The method takes full account of symmetry and is applicable to any crystal or molecule. Force constants can be specified in terms of any two-atom “bonds” or three-atom angles. The essential steps in the computer program are: (1) Locate all atoms in the unit cell or molecule and compute displacement vectors for each internal coordinate; (2) Convert the basis of the force constants from bonds and angles to cartesian displacements; (3) Construct the full-matrix irreducible representations of the point or factor group in question, using appropriate symmetry matrices and polynomial basis functions; (4) Derive the symmetry coordinates in terms of cartesian displacements using the projection/transfer-operator technique; (5) construct secular equations for each species with Wilson's f–g method; (6) solve for frequencies and atomic motions; and (7) use simple models of infrared and Raman intensities to calculate spectra.     

云南兰坪富锶文石的振动光谱特征

为进一步研究富锶文石的晶体结构、颜色成因和矿物成因，采用振动光谱、EMPA和SEM对富锶文石进行了测试和分析。红外光谱、拉曼光谱测试分析表明，由于富锶文石存在着锶与钙的类质同像现象，其红外光谱和拉曼光谱中CO3^2-的v1、v2、v3、v4种振动模式波数介于文石和碳酸锶矿之间，与文石、碳酸锶矿等文石型碳酸盐振动光谱特征一致，并且文石的v4存在分裂峰。EMPA面扫描发现Ca^2 、Sr^2 的分布与富锶文石内部环带的分布存在一致性。由于CuO含量较少，分布规律不明显。SEM分析发现在环带之间存在空隙，各环带中富锶文石结晶程度和晶体表面特征不一，不同颜色和形态的环带反复生长。生长过程中，流体成分中各种元素的含量、温度和压力的变化造成不同颜色环带的形成，反映了该富锶文石是经过不同时期沉积形成的。     

Thermodynamic properties of chlorite and berthierine derived from calorimetric measurements

In the context of the deep waste disposal, we have investigated the respective stabilities of two iron-bearing clay minerals: berthierine ISGS from Illinois [USA; (Al_0.975FeIII_0.182FeII_1.422Mg_0.157Li_0.035Mn_0.002)(Si_1.332Al_0.668)O₅(OH)₄] and chlorite CCa-2 from Flagstaff Hill, California [USA; (Si_2.633Al_1.367)(Al_1.116FeIII_0.215Mg_2.952FeII_1.712Mn_0.012Ca_0.011)O₁₀(OH)₈]. For berthierine, the complete thermodynamic dataset was determined at 1 bar and from 2 to 310 K, using calorimetric methods. The standard enthalpies of formation were obtained by solution-reaction calorimetry at 298.15 K, and the heat capacities were measured by heat-pulse calorimetry. For chlorite, the standard enthalpy of formation is measured by solution-reaction calorimetry at 298.15 K. This is completing the entropy and heat capacity obtained previously by Gailhanou et al. (Geochim Cosmochim Acta 73:4738–4749, 2009) between 2 and 520 K, by using low-temperature adiabatic calorimetry and differential scanning calorimetry. For both minerals, the standard entropies and the Gibbs free energies of formation at 298.15 K were then calculated. An assessment of the measured properties could be carried out with respect to literature data. Eventually, the thermodynamic dataset allowed realizing theoretical calculations concerning the berthierine to chlorite transition. The latter showed that, from a thermodynamic viewpoint, the main factor controlling this transition is probably the composition of the berthierine and chlorite minerals and the nature of the secondary minerals rather than temperature.     

Aluminum speciation, vibrational entropy and short-range order in calcium aluminosilicate glasses

The heat capacity and vibrational entropy of a calcium aluminate and three peraluminous calcium aluminosilicate glasses have been determined from 2 to 300 K by heat-pulse relaxation calorimetry. Together with previous adiabatic data for six other glasses in the system CaO-Al₂O₃-SiO₂, these results have been used to determine partial molar heat capacities and entropies for five species namely, SiO₂, CaO and three different sorts of Al₂O₃ in which Al is 4-, 5- and 6-fold coordinated by oxygen. Given the determining role of oxygen coordination on low-temperature heat capacity, the composition independent entropies found for SiO₂ and CaO indicate that short-range order around Si and Ca is not sensitive to aluminum speciation up to the highest fraction of 25% observed for ^VAl by NMR spectroscopy. Because of the higher room-temperature vibrational entropy of ^IVAl₂O₃ (72.8 J/mol K) compared to ^VAl₂O₃ (48.5 J/mol K), temperature-induced changes from ^IVAl to ^VAl give rise to a small negative contribution of the order of 1 J/mol K to the partial molar configurational heat capacity of Al₂O₃ in melts. Near 0 K, pure SiO₂ glass distinguishes itself by the importance of the calorimetric boson peak. On a g atom basis, the maximum of this peak varies with the composition of calcium aluminosilicate glasses by a factor of about 2. It does not show smooth variations, however, either as a function of SiO₂ content, at constant CaO/Al₂O₃ ratio, or as a function of Al₂O₃ content, at constant SiO₂ content.